X-linked immunoglobulin deficiency with normal or elevated IgM (X- linked hyper IgM, HIGMS-1) is a primary immune deficiency disease that results from defects in the genenfor CD40 ligand (CD40L). CD40L is a 39 kDa glycoprotein expressed transiently on the surface of activated T cells which delivers contact-dependent signals to B cells and to other CD40-expressing cells. The loss of interaction between CD40 and its ligand in HIGMX-1 interferes with the numerous functions of this ligand pair and explains the profound immune dificiency state in this disease. Administration of intravenous immunoglobulin improves the patients' quality of life but does not prevent the fatal complication of lymphoproliferative disease. Towards achieving a genetic therapy for HIGMS-1, the aim of this proposal is to develop a model for gene transfer of CD40L in this disease. Normal human CD40L cDNa will be expressed under the regulatory control of the human IL-2 promoter. (1) The capacity of the CD40L gene transfer construct to provide functional expression of CD40L will be assessed by generating a transgenic mouse expressing the construct and crossing it with a CD40L-deficient mouse. (2) Reconstitution of CD40L expression and function in CD40L-deficient mice will be evaluated by transplatation of bone narrow cells from CD40L-transgenic/CD40L-deficient mice into lethally irradiated CD40L-deficient mice. (3) The capacity of recombinant adeno- associated virus to transduce CD40L-deficient hematopoietic stem cells and reconstitute CD40L expression and function in lymphocyte progeny will be investigated. The results of this study: will assess the capacity to reconstitute CD40L expression and function in a murine model of HIGMX-1; will evaluate the transfer of a gene under the regulatory control of a T cell activation-dependent promoter using adeno-asoociated virus; and will determine the fraction of CD44+T cells required to express normal CD40L in order to restore immune function.